1. Field of the Invention
The invention relates to a glass-ceramic having at least two crystal phases which can be employed, in particular, as dielectric in the high-frequency range (frequency>200 MHz), in particular in the gigahertz range (frequency f>1 GHz). The invention further relates to a process for producing a glass-ceramic having at least two crystal phases and to its use.
2. Description of Related Art
Specific materials which have a very high relative permittivity ∈ at a very low dielectric loss (tan δ) are required for a series of applications in the high-frequency range. To avoid detuning by the body of a user in close proximity, known as “body loading”, dielectric charging of antennae, filters and other devices is of particular importance. Here, it is necessary to have dielectrics which have a high relative permittivity of ∈≧15 and a low dielectric loss (tan δ) of not more than 10−2 and preferably lower in the high-frequency range. Furthermore, the temperature dependence of the resonance frequency τf should be very low. Finally, such a material should be able to be processed in a very simple and inexpensive way in order to make it possible to obtain near net shape cheaply.
A glass-ceramic system is essentially known from the prior art. This is a BiNbO4 system which is disclosed in Mirsaneh et al., “Cirularly Polarized Dielectric-Loaded Antennas: Current Technology and Future Challanges”, Adv. Funct. Materials 18, (2008), pp. 2293-2300, for use in dielectrically loaded antennae for the gigahertz range. This material can be used for producing the two forms of antennae which are mainly used, viz. circularly polarized DLA helix antennae (D-LQH antennae) and square patch antennae. For this purpose, a glass having the composition 30 mol % of Bi2O3, 30 mol % of Nb2O5, 30 mol % of B2O3 and 10 mol % of SiO2 is firstly melted in the conventional way at 1250° C. for two hours. This glass was poured into cylindrical molds, annealed at 500-520° C. and slowly cooled to room temperature. This was followed by crystallization at various temperatures in the range from 600° C. to 1000° C. In the case of a heat treatment at 960° C., a relative permittivity ∈ of 15 with a quality factor Q·fo of 15 000 GHz and a temperature coefficient of the resonance frequency τf of −80 ppm/K is reported as optimal value for antenna applications. Mainly orthorhombic BiNbO4 was characterized as crystalline phase here.
Glass-ceramics having at least one mixed crystal phase are known as dielectric in the high-frequency range from the patent application DE 10 2010 012 524.5-45. The glass-ceramics have at least the following constituents (in mol % on an oxide basis):
SiO25-50Al2O30-20B2O30-25BaO0-25TiO210-60 RE2O3 5-35,where Ba can be partly, preferably to an extent of up to 10%, replaced by Sr, Ca, Mg, where RE is lanthanum, another lanthanide or yttrium and where Ti can be partly, preferably to an extent of up to 10%, replaced by Zr, Hf, Y, Nb, V, Ta. The absolute value of the temperature dependence of the resonance frequency τf of these glass-ceramics is preferably not more than 200 ppm/K, preferably not more than 50 ppm/K, particularly preferably not more than 10 ppm/K.
In addition, there is a series of sintered ceramic materials (cf. U.S. Pat. No. 6,184,845 B1, US 2007/063902 A1). Here, a sintered ceramic material based on zirconate titanate or based on tin zirconate titanate having a relative permittivity of about 36 is reported as dielectric material for the ceramic core of a dielectrically charged D-LQH antenna. The material is said to be produced by extrusion or pressing and subsequent sintering.
Further sintered materials are reported in the article by M. T. Sebastian et al., “Low loss dielectric materials for LTCC applications”, International Materials Reviews, Vol. 53 (2008), pp. 57-90. Even though these materials are sometimes referred to as “glass-ceramics”, they are sintered materials since they have been produced by sintering of a mixture of vitreous and crystalline powders and not by ceramicization of a starting glass.
Dielectrics produced by sintering have a series of disadvantages: for instance, every sintering process is always associated with a certain shrinkage which leads to geometric inaccuracies and corresponding final machining. Furthermore, every sintering process leaves a certain residual porosity which is disadvantageous in the case of metallization of the surface. The metal penetrates into the pores and increases the dielectric loss of the dielectric.
In addition, the production of sintered materials is fundamentally relatively complicated and expensive.
Furthermore, La—Ti oxide compounds which have a low dielectric loss (tan ) have been described by: J. Takahashi et al., Jpn. J. Appl. Phys, 32, 4327 (1993). These materials are characterized by their dielectric function , which is usually made up of a large real part ′ and a small imaginary part ″. In the GHz spectral range, the dielectric loss (tan ) is characterized by a dimensionless quality factor Q, Q=′/″=1/tan , or by the Q factor multiplied by the frequency, Q·f, which is usually reported in GHz.
A further possible way of characterizing the material is to determine the temperature dependence of the resonance frequency τf in the GHz range, which indicates the change in the resonance frequency at a particular material geometry as a function of the temperature. When f0 is the resonance frequency at a particular material geometry, the temperature dependence of the resonance frequency in the vicinity of room temperature can be expressed as follows: f0(T)≈f0(TRT)+τf(T−TRT) where τf is defined as follows
      τ    f    =                    1                  f          0                    ·                        ∂                      f            0                                    ∂          T                      ⁢          |              T        =                  T          RT                      .  
A resonance structure, e.g. a filter or an antenna, made of a material which has no temperature dependence of the resonance frequency τf in the GHz range (τf≈0) accordingly displays no change in its resonance frequency at changing ambient temperatures.
To achieve this in the case of ceramic materials, ceramic starting components having opposite signs of the temperature dependence of the resonance frequencies τf are mixed and balanced so that the resulting ceramic material has τf≈0 (J. Takahashi et al., Jpn. J. Appl. Phys, 32, 4327 (1993)). Here, it is proposed that the starting components 10 mol % of La2Ti2O7 and 90 mol % of La4Ti9O24 be mixed so that the ceramic material produced therefrom has τf≈0. Glass-ceramics which have a negligible temperature dependence of the resonance frequency τf in the GHz range are not known.